The Scale of Pervasive Developmental Disorder in Mentally Retarded Persons (PDD-MRS) is described. The PDD-MRS is a simple classification and screening instrument devised for identification of autistic disorders (of the entire spectrum) in persons with mental retardation from mild to profound levels, age-range 2-55 years. The norms of the scale are based on the research protocols of 1230 Dutch persons with mental retardation. The scale's sensitivity for the entire normative sample was found to be 92.4%; calculated separately for persons at all levels of mentally retarded functioning, male and female persons, speaking and non-speaking persons and five age categories, the sensitivity figures range between 87.0 and 100.0%. The specificity of the scale is also 92.4%; for the forementioned subgroups separately, the specificity figures range between 84.6 and 95.5%. Roughly similar values for sensitivity and specificity were found when using the scale with severely visually impaired/blind persons; severely hearing-impaired/deaf persons; persons with Down syndrome; male persons with fragile X syndrome. The original version of the PDD-MRS dates from 1990; since then the scale has been widely used in the Netherlands and Belgium. The PDD-MRS should be regarded as a useful instrument for identifying PDD in persons with mental retardation.